Can you explain the HttpURLConnection connection process

Question

I am using HTTPURLConnection to connect to a web service  I know how to use HTTPURLConnection but I want to understand how it works  Basically  I want to know the following    On which point does HTTPURLConnection try to establish a connection to the given URL  On which point can I know that I was able to successfully establish a connection  Are establishing a connection and sending the actual request done in one step method call  What method is it  Can you explain the function of getOutputStream and getInputStream in layman s term  I notice that when the server I m trying to connect to is down  I get an Exception at getOutputStream  Does it mean that HTTPURLConnection will only start to establish a connection when I invoke getOutputStream  How about the getInputStream  Since I m only able to get the response at getInputStream  then does it mean that I didn t send any request at getOutputStream yet but simply establishes a connection  Do HttpURLConnection go back to the server to request for response when I invoke getInputStream  Am I correct to say that openConnection simply creates a new connection object but does not establish any connection yet  How can I measure the read overhead and connect overhead

User · Answer

On which point does HTTPURLConnection try to establish a connection to the given URL   It s worth clarifying  there s the  UrlConnection  instance and then there s the underlying Tcp Ip SSL socket connection  2 different concepts   The  UrlConnection  or  HttpUrlConnection  instance is synonymous with a single HTTP page request  and is created when you call url openConnection      But if you do multiple url openConnection   s from the one  url  instance then if you re lucky  they ll reuse the same Tcp Ip socket and SSL handshaking stuff   which is good if you re doing lots of page requests to the same server  especially good if you re using SSL where the overhead of establishing the socket is very high   See  HttpURLConnection implementation

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String message   URLEncoder encode  my message    UTF-8     try          instantiate the URL object with the target URL of the resource to        request     URL url   new URL  http   www example com comment            instantiate the HttpURLConnection with the URL object - A new        connection is opened every time by calling the openConnection        method of the protocol handler for this URL         1  This is the point where the connection is opened      HttpURLConnection connection    HttpURLConnection  url              openConnection           set connection output to true     connection setDoOutput true          instead of a GET  we re going to send using method  POST      connection setRequestMethod  POST            instantiate OutputStreamWriter using the output stream  returned        from getOutputStream  that writes to this connection         2  This is the point where you ll know if the connection was        successfully established  If an I O error occurs while creating        the output stream  you ll see an IOException      OutputStreamWriter writer   new OutputStreamWriter              connection getOutputStream             write data to the connection  This is data that you are sending        to the server        3  No  Sending the data is conducted here  We established the        connection with getOutputStream     writer write  message     message           Closes this output stream and releases any system resources        associated with this stream  At this point  we ve sent all the        data  Only the outputStream is closed at this point  not the        actual connection     writer close           if there is a response code AND that response code is 200 OK  do        stuff in the first if block     if  connection getResponseCode      HttpURLConnection HTTP OK               OK             otherwise  if any other status code is returned  or no status            code is returned  do stuff in the else block       else              Server returned HTTP error code          catch  MalformedURLException e                 catch  IOException e                   The first 3 answers to your questions are listed as inline comments  beside each method  in the example HTTP POST above    From getOutputStream      Returns an output stream that writes to this connection    Basically  I think you have a good understanding of how this works  so let me just reiterate in layman s terms  getOutputStream basically opens a connection stream  with the intention of writing data to the server  In the above code example  message  could be a comment that we re sending to the server that represents a comment left on a post  When you see getOutputStream  you re opening the connection stream for writing  but you don t actually write any data until you call writer write  message     message     From getInputStream        Returns an input stream that reads from this open connection  A SocketTimeoutException can be thrown when reading from the returned input stream if the read timeout expires before data is available for read    getInputStream does the opposite  Like getOutputStream  it also opens a connection stream  but the intent is to read data from the server  not write to it  If the connection or stream-opening fails  you ll see a SocketTimeoutException      How about the getInputStream  Since I m only able to get the response at getInputStream  then does it mean that I didn t send any request at getOutputStream yet but simply establishes a connection    Keep in mind that sending a request and sending data are two different operations  When you invoke getOutputStream or getInputStream url openConnection    you send a request to the server to establish a connection  There is a handshake that occurs where the server sends back an acknowledgement to you that the connection is established  It is then at that point in time that you re prepared to send or receive data  Thus  you do not need to call getOutputStream to establish a connection open a stream  unless your purpose for making the request is to send data   In layman s terms  making a getInputStream request is the equivalent of making a phone call to your friend s house to say  Hey  is it okay if I come over and borrow that pair of vice grips   and your friend establishes the handshake by saying   Sure  Come and get it   Then  at that point  the connection is made  you walk to your friend s house  knock on the door  request the vice grips  and walk back to your house   Using a similar example for getOutputStream would involve calling your friend and saying  Hey  I have that money I owe you  can I send it to you   Your friend  needing money and sick inside that you kept it for so long  says  Sure  come on over you cheap bastard   So you walk to your friend s house and  POST  the money to him  He then kicks you out and you walk back to your house   Now  continuing with the layman s example  let s look at some Exceptions  If you called your friend and he wasn t home  that could be a 500 error  If you called and got a disconnected number message because your friend is tired of you borrowing money all the time  that s a 404 page not found  If your phone is dead because you didn t pay the bill  that could be an IOException   NOTE  This section may not be 100  correct  It s intended to give you a general idea of what s happening in layman s terms    Question  5   Yes  you are correct that openConnection simply creates a new connection object but does not establish it  The connection is established when you call either getInputStream or getOutputStream   openConnection creates a new connection object  From the URL openConnection javadocs      A new connection is opened every time by calling the openConnection method of the protocol handler for this URL    The connection is established when you call openConnection  and the InputStream  OutputStream  or both  are called when you instantiate them   Question  6   To measure the overhead  I generally wrap some very simple timing code around the entire connection block  like so   long start   System currentTimeMillis    log info  Time so far       new Long System currentTimeMillis   - start         run the above example code here log info  Total time to send receive data       new Long System currentTimeMillis   - start       I m sure there are more advanced methods for measuring the request time and overhead  but this generally is sufficient for my needs   For information on closing connections  which you didn t ask about  see In Java when does a URL connection close

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I went through the exercise to capture low level packet exchange  and found that network connection is only triggered by operations like getInputStream  getOutputStream  getResponseCode  getResponseMessage etc    Here is the packet exchange captured when I try to write a small program to upload file to Dropbox     Below is my toy program and annotation         Create a connection LOCAL object         the openConnection   function DOES NOT initiate        any packet exchange with the remote server                 The configurations only setup the LOCAL        connection object properties              HttpURLConnection connection    HttpURLConnection  dst openConnection        connection setDoOutput true       connection setRequestMethod  POST             headers setup     byte   testContent    0x32  0x32                   This triggers packet exchange with the remote        server to create a link  But writing flushing        to a output stream does not send out any data                 Payload are buffered locally              try  BufferedOutputStream outputStream   new BufferedOutputStream connection getOutputStream               outputStream write testContent           outputStream flush                          Trigger payload sending to the server         Client get ALL responses  including response code         message  and content payload               int responseCode   connection getResponseCode        System out println responseCode           Here no further exchange happens with remote server  since        the input stream content has already been buffered        in previous step             try  InputStream is   connection getInputStream              Scanner scanner   new Scanner is           StringBuilder stringBuilder   new StringBuilder            while  scanner hasNextLine              stringBuilder append scanner nextLine    append System lineSeparator                                     Trigger the disconnection from the server              String responsemsg   connection getResponseMessage        System out println responsemsg       connection disconnect

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On which point does HTTPURLConnection try to establish a connection to the given URL   On the port named in the URL if any  otherwise 80 for HTTP and 443 for HTTPS  I believe this is documented   On which point can I know that I was able to successfully establish a connection   When you call getInputStream   or getOutputStream   or getResponseCode   without getting an exception   Are establishing a connection and sending the actual request done in one step method call  What method is it   No and none   Can you explain the function of getOutputStream   and getInputStream   in layman s term   Either of them first connects if necessary  then returns the required stream   I notice that when the server I m trying to connect to is down  I get an Exception at getOutputStream    Does it mean that HTTPURLConnection will only start to establish a connection when I invoke getOutputStream    How about the getInputStream    Since I m only able to get the response at getInputStream    then does it mean that I didn t send any request at getOutputStream   yet but simply establishes a connection  Do HttpURLConnection go back to the server to request for response when I invoke getInputStream     See above   Am I correct to say that openConnection   simply creates a new connection object but does not establish any connection yet   Yes   How can I measure the read overhead and connect overhead   Connect  take the time getInputStream   or getOutputStream   takes to return  whichever you call first  Read  time from starting first read to getting the EOS

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Tim Bray presented a concise step-by-step  stating that openConnection   does not establish an actual connection  Rather  an actual HTTP connection is not established until you call methods such as getInputStream   or getOutputStream     http   www tbray org ongoing When 201x 2012 01 17 HttpURLConnection

[java] Can you explain the HttpURLConnection connection process?

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